The invention disclosed and claimed herein pertains to a system for optical heterodyne imaging. More particularly, the invention pertains to an optical heterodyne imaging system for viewing an object through a medium or environment containing diffused refractive particles. Even more particularly, the invention pertains to such systems which illuminate an object to be viewed with coherent light having a very short coherence light to overcome the degrading effects of forward scatter and back scatter.
The range at which an optical system viewing an object through a diffusive medium may form an image of the object is substantially limited by the phenomenon of forward scatter, and if the object is illuminated by light projected from the viewing system, by the phenomenon of back scatter. A diffusive medium is a medium such as fog or seawater which contains refractive diffused particulate matter. Forward scatter is the scattering of light transmitted from a viewed object to a viewing system as the result of random refractions by the particles, where some of the scattered light is sensed by the viewing system. While some of the light transmitted from the object will reach the viewing system without interacting with any particles, and will therefore provide a clear image of the object, the sensed scattered light will degrade the image at the photodetector of the system. If the viewing system is more than a limited distance from the object, the scattered light will cause the image to be totally unviewable.
Forward scatter generally becomes a significant problem at distances which exceed 10 attenuation lengths, and makes an object completely unviewable at 15 attenuation lengths. For example, if seawater contains a large concentration of certain refractive microorganisms, a conventional viewing device such as a television camera would be unable to distinguish an object located more than 15 attenuation lengths away.
Back scatter is the scattering of light projected from a source of light by refractive particles of a diffusive medium, where some of the scattered light is directed back toward the source. It is clear that back scatter will further degrade an image of an object which is viewed by a system which must provide illumination for the object.
Some techniques are presently available for overcoming the detrimental effects of back scatter in a diffusive medium such as seawater. For example, volume scanning or range gating may be employed. As far as is known, the only means for overcoming forward scatter in a dynamic diffusive medium, i.e., a medium in which the particles are in continuous random motion, is an invention by Applicant, disclosed in a previously filed patent application entitled "Optical Heterodyne System for Imaging in a Dynamic Diffusive Medium", filed with the U.S. Patent and Trademark Office under Ser. No. 930,283 on Aug. 2, 1978. As far as is known, no conventional systems or techniques are presently available for overcoming forward scatter in a diffusive medium in which the particles do not move.